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Merrin, J.

Publications and source records attributed to Merrin, J..

14 recordsLinked to original sources

Simulation-Based Inference of Cell Migration Dynamics in Complex Spatial Environments

To assess cell migration in complex spatial environments, microfabricated chips, such as mazes and pillar forests, are routinely used to impose spatial and mechanical constraints, and cell trajectories are followed within these structures by advanced imaging techniques. In systems mechanobiology, computational models serve as essential tools to uncover how physical geometry influences intracellular dynamics; however, decoding such complex behaviors requires advanced inference techniques. Here, we integrated experimental observations of dendritic cell migration in a geometrically constrained microenvironment into a Cellular Potts model. We demonstrated that these spatial constraints modulate the motility dynamics, including speed and directional changes. We show that classical summary statistics, such as mean squared displacement and turning angle distributions, can resolve key mechanistic features but fail to extract richer spatiotemporal patterns, limiting accurate parameter inference. To solve this, we applied neural posterior estimation with in-the-loop learning of summary features. This learned summary representation of the data enables robust and flexible parameter inference, providing a data-driven framework for model calibration and advancing quantitative analysis of cell migration in structured microenvironments.

cell biology↗

Substrate heterogeneity promotes cancer cell dissemination

While tumor malignancy has been extensively studied under the prism of genetic and epigenetic heterogeneity, tumor cell states also critically depend on reciprocal interactions with the microenvironment. This raises the hitherto untested possibility that heterogeneity of the untransformed tumor stroma can actively fuel malignant progression. As biological heterogeneity is inherently difficult to control, we adopted a reductionist approach and let tumor cells invade micro-engineered environments harboring obstacles with precision-controlled geometry. We find that not only the presence of obstacles, but more surprisingly their spatial disorder, causes a drastic shift from a collective to a single-cell mode of invasion - comparable in strength to cadherin loss. Combining live-imaging and perturbation experiments with minimal biophysical modeling, we demonstrate that cell detachments result both from local geometrical constraints and a global integration of spatial disorder over time. We show that different types of microenvironments map onto different universality classes of invasion dynamics - homogeneous substrates follow Kardar-Parisi-Zhang (KPZ) scaling, while disordered ones exhibit exponents consistent with KPZ with quenched disorder (KPZq). Our findings highlight generic physical principles for how the mode of cancer cell invasion depends on environmental heterogeneity, with potential implications to understand tumor evolution in vivo.

biophysics↗

Mechanical Centrosome Fracturing during Cell Navigation

The centrosome is the primary microtubule orchestrator in most eukaryotic cells, nucleating and anchoring microtubules that grow radially and exert forces on cargos. At the same time, mechanical stresses from the microenvironment and cellular shape changes compress and bend microtubules. Yet, centrosomes are membrane-less organelles, raising the question of how centrosomes withstand mechanical forces. Here we discover that centrosomes in non-dividing cells can mechanically fracture. We reveal that centrosomes experience mechanical deformations during microenvironmental confinement and navigational pathfinding by motile cells. Coherence of the centrosome is maintained by Dyrk3, preventing fracturing by mechanical forces. Centrosome fracturing impedes cellular function by generating coexisting microtubule organizing centers that compete during path navigation and thereby cause cellular entanglement in the microenvironment. Our findings show that non-dividing cells actively maintain the integrity of the centrosome to withstand mechanical forces. Given that almost all cells in multicellular organism experience forces, these results suggest that centrosome stability preservation is fundamental during development, tissue maintenance, immunology, and disease. One-Sentence SummaryPathfinding during cellular motility causes centrosome breakage counteracted by Dyrk3 activity.

cell biology↗

Global coordination of protrusive forces in migrating immune cells

Efficient immune responses rely on the capacity of leukocytes to traverse diverse and complex tissues. To meet such changing environmental conditions, leukocytes usually adopt an amoeboid configuration, utilizing their forward-positioned nucleus as a probe to identify and follow the path of least resistance among pre-existing pores. We show that in dense environments, where even the largest pores preclude free passage, leukocytes switch polarity and position their nucleus behind centrosome and organelles. In this mesenchymal configuration, local compression of the cell body triggers assembly of a central F-actin pool, located between cell front and nucleus. Central actin pushes outward to transiently dilate a path for organelles and nucleus. Pools of central and front actin are tightly coupled and experimental depletion of the central pool enhances actin accumulation and protrusion formation at the cell front. Although this shifted balance speeds up cells in permissive environments, migration in restrictive environments is impaired, as the unleashed leading edge dissociates from the trapped cell body. Our findings establish an actin regulatory loop that balances path dilation with advancement of the leading edge to maintain cellular coherence.

cell biology↗

Hijacked Immune Cells Traverse Microenvironmental Barriers by Positioning and Pushing their Intracellular Parasite Cargo

Cell migration is fundamental to development, homeostasis, and immunity, and requires cells to traverse space-limited microenvironments by deforming intracellular organelles such as the nucleus. Although nuclear mechanics are established as a rate-limiting factor during confined migration, how cells transport bulky foreign cargo remains unclear. Here, using Toxoplasma gondii, a disease-causing parasite that hijacks motile immune cells for dissemination while replicating into large intracellular assemblies, we discover that parasitic cargoes exceed the size and stiffness of host nuclei, and therefore constitute a primary bottleneck for migration. We uncover a mechanoadaptive response to parasite size, characterized by size-dependent intracellular repositioning, myosin-II contractility, and bleb-based protrusions. This reprogramming of forces and subcellular architecture enables the transport of oversized, mechanically rigid cargo through space-limited microenvironments. These findings establish a mechanobiological principle of how cells size-dependently adapt forces and their subcellular architecture to accommodate intracellular objects that challenge the physical limits of cell migration.

cell biology↗

Fabrication of elastomeric stencils for patterned stem cell differentiation

Stem cell differentiation with controlled geometry results in reproducible pattern formation. In contrast to constraining differentiating cells on micropatterned surfaces, we initialise colony formation using elastomeric stencils that adhere to culture dishes and create microwells with defined sizes and shapes. After colony formation, stencils are removed to allow colony growth and cell migration. Stencil fabrication involves mould production by photolithography followed by replica-moulding polydimethylsiloxane (PDMS). This approach produces reproducible two-dimensional organoids tailored for quantitative studies of growth control and pattern formation.

developmental biology↗

Self-organised pattern formation in the developing neural tube by a temporal relay of BMP signalling

Developing tissues interpret dynamic changes in morphogen activity to generate cell type diversity. To quantitatively study BMP signalling dynamics in the vertebrate neural tube, we developed a new ES cell differentiation system tailored for growing tissues. Differentiating cells form striking self-organised patterns of dorsal neural tube cell types driven by sequential phases of BMP signalling that are observed both in vitro and in vivo. Data-driven biophysical modelling showed that these dynamics result from coupling fast negative feedback with slow positive regulation of signalling by the specification of an endogenous BMP source. Thus, in contrast to relays that propagate morphogen signalling in space, we uncover a BMP signalling relay that operates in time. This mechanism allows rapid initial concentrationsensitive response that is robustly terminated, thereby regulating balanced sequential cell type generation. Altogether, our study provides an experimental and theoretical framework to understand how signalling dynamics are exploited in developing tissues.

developmental biology↗

Adaptive Pathfinding by Nucleokinesis during Amoeboid Migration

Motile cells moving in multicellular organisms encounter microenvironments of locally heterogeneous mechanochemical composition. Individual compositional parameters like chemotactic signals, adhesiveness, and pore sizes are well known to be sensed by motile cells, providing individual guidance cues for cellular pathfinding. However, motile cells encounter diverse mechanochemical signals at the same time, raising the question of how cells respond to locally diverse and potentially competing signals on their migration routes. Here, we reveal that motile amoeboid cells require nuclear repositioning, termed nucleokinesis, for adaptive pathfinding in heterogeneous mechanochemical microenvironments. Using mammalian immune cells and the amoeba Dictyostelium discoideum, we discover that frequent, rapid and long-distance nucleokinesis is a basic component of amoeboid pathfinding, enabling cells to reorientate quickly between locally competing cues. Amoeboid nucleokinesis comprises a two-step cell polarity switch and is driven by myosin II-forces, sliding the nucleus from a losing to the winning leading edge to re-adjust the nuclear to the cellular path. Impaired nucleokinesis distorts fast path adaptions and causes cellular arrest in the microenvironment. Our findings establish that nucleokinesis is required for amoeboid cell navigation. Given that motile single-cell amoebae, many immune cells, and some cancer cells utilize an amoeboid migration strategy, these results suggest that amoeboid nucleokinesis underlies cellular navigation during unicellular biology, immunity, and disease.

cell biology↗

Adhesion-induced cortical flows pattern E-cadherin-mediated cell contacts

Metazoan development relies on the formation and remodeling of cell-cell contacts. Dynamic reorganization of adhesion receptors and the actomyosin cell cortex in space and time play a central role in cell-cell contact formation and maturation. Yet, how this process is mechanistically achieved remains unclear. Here, by building a biomimetic assay composed of progenitor cells adhering to supported lipid bilayers functionalized with E-cadherin ectodomains, we show that cortical Actin flows, driven by the depletion of Myosin-2 at the cell contact center, mediate the dynamic reorganization of adhesion receptors and cell cortex at the contact. E-cadherin-dependent downregulation of the small GTPase RhoA at the forming contact leads to both a depletion of Myosin-2 and a decrease of F-actin at the contact center. This depletion of Myosin-2 causes centrifugal F-actin flows, leading to further accumulation of F-actin at the contact rim and the progressive redistribution of E-cadherin from the contact center to the rim. Eventually, this combination of actomyosin downregulation and flows at the contact determine the characteristic molecular organization, with E-cadherin and F-actin accumulating at the contact rim, where they are needed to mechanically link the contractile cortices of the adhering cells.

cell biology↗

Synchronization in collectively moving inanimate and living active matter

Regardless of whether one considers swarming insects, flocking birds, or bacterial colonies, collective motion arises from the coordination of individuals and entails the adjustment of their respective velocities. In particular, in close confinement, such as those encountered by dense cell populations during development or regeneration, collective migration can only arise coordinately. Yet, how individuals unify their velocities is often not understood. Focusing on a finite number of cells in circular confinements, we identify waves of polymerizing actin that function as a pacemaker governing the speed of individual cells. We show that the onset of collective motion coincides with the synchronization of the wave nucleation frequencies across the population. Employing a simpler and more readily accessible mechanical model system of active spheres, we identify the essential requirements to reach the corresponding collective state, i.e. the synchronization of the individuals internal oscillators. The mechanical toy experiment illustrates that the global synchronous state is achieved by nearest neighbor coupling. We suggest by analogy that local coupling and the synchronization of actin waves are essential for emergent, self-organized motion of cell collectives.

biophysics↗

Sinking the way: a dual role for CCR7 in collective leukocyte migration

Immune responses crucially rely on the rapid and coordinated locomotion of leukocytes. While it is well established that single-cell migration is often guided by gradients of chemokines and other chemoattractants, it remains poorly understood how such gradients are generated, maintained and modulated. Combining experiment and theory on leukocyte chemotaxis guided by the G protein-coupled receptor (GPCR) CCR7, we demonstrate that in addition to its role as the sensory receptor that steers migration, CCR7 also acts as a generator and modulator of chemotactic gradients. Upon exposure to the CCR7 ligand CCL19, dendritic cells (DCs) effectively internalize the receptor and ligand as part of the canonical GPCR-desensitization response. We show that CCR7 internalization also acts as an effective sink for the chemoattractant, thereby dynamically shaping the spatio-temporal distribution of the chemokine. This mechanism drives complex collective migration patterns, enabling DCs to create or sharpen chemotactic gradients. We further show that these self-generated gradients can sustain the long-range guidance of DCs, adapt collective migration patterns to the size and geometry of the environment, as well as provide a guidance cue for other co-migrating cells. Such dual role of CCR7 as a GPCR that both senses and consumes its ligand can thus provide a novel mode of cellular self-organization.

cell biology↗

Mechanosensitivity of amoeboid cells crawling in 3D

Efficient immune-responses require migrating leukocytes to be in the right place at the right time. When crawling through the body amoeboid leukocytes must traverse complex three-dimensional tissue-landscapes obstructed by extracellular matrix and other cells, raising the question how motile cells adapt to mechanical loads to overcome these obstacles. Here we reveal the spatio-temporal configuration of cortical actin-networks rendering amoeboid cells mechanosensitive in three-dimensions, independent of adhesive interactions with the microenvironment. In response to compression, Wiskott-Aldrich syndrom protein (WASp) assembles into dot-like structures acting as nucleation sites for actin spikes that in turn push against the external load. High precision targeting of WASp to objects as delicate as collagen fibers allows the cell to locally and instantaneously deform its viscoelastic surrounding in order to generate space for forward locomotion. Such pushing forces are essential for fast and directed leukocyte migration in fibrous and cell-packed tissues such as skin and lymph nodes. In BriefWASp-driven actin spikes counter compressive loads of crowded tissue-landscapes.

cell biology↗

Adhesion-free cell migration by topography-based force transduction

Eukaryotic cells migrate by coupling the intracellular force of the actin cytoskeleton to the environment. While force-coupling is usually mediated by transmembrane adhesion receptors, especially these of the integrin family, amoeboid cells like leukocytes can migrate extremely fast despite very low adhesive forces1. We show that leukocytes cannot only migrate under low adhesion but indeed can transduce forces in the complete absence of transmembrane force coupling. When confined within three-dimensional environments, they use the topographic features of the substrate to propel themselves. Here, the retrograde flow of the actin cytoskeleton follows the texture of the substrate, creating shear forces sufficient to drive deformations towards the back of the cell. Notably, adhesion dependent and adhesion independent migration are not exclusive but rather variants of the same principle of coupling retrograde actin flow to the environment and thus can potentially operate simultaneously. As adhesion free migration is independent of the chemical composition of the environment it renders cells completely autonomous in their locomotive behavior.

cell biology↗

Microtubules coordinate protrusion-retraction dynamics in migrating dendritic cells

Cells navigating through tissues face a fundamental challenge: while multiple cellular protrusions explore different paths through the complex geometry of an interstitial matrix the cell needs to avoid becoming too long or ramified, which might ultimately lead to a loss of physical coherence. How a cell surveys its own shape to inform the actomyosin system to retract entangled or stretched protrusions is not understood. Here, we demonstrate that spatially distinct microtubule (MT) dynamics regulate amoeboid cell migration by locally specifying the retraction of explorative protrusions. In migrating dendritic cells (DCs), the microtubule organizing center (MTOC) guides the path through a three dimensional (3D) interstitium and local MT depolymerization in protrusions remote from the MTOC triggers myosin II dependent contractility via the RhoA exchange factor Lfc. Depletion of Lfc leads to aberrant myosin localization, thereby causing two effects that rate-limit locomotion: i) impaired cell edge coordination during path-finding and ii) defective adhesion-resolution. Such compromised cell shape control is particularly hindering when cells navigate through geometrically complex microenvironments, where it leads to entanglement and ultimately fragmentation of the cell body. Our data demonstrate that MTs control cell shape and coherence by locally controlling protrusion-retraction dynamics of the actomyosin system.

cell biology↗